1. Field of the Invention
The present invention relates to a method for producing a grain-oriented electrical steel sheet having a high magnetic flux density.
2. Description of the Prior Art
Grain-oriented electrical steel sheet is a soft magnetic material composed of crystal grains having a so called Goss texture, expressed by {110} &lt;001&gt; by the Miller index in which the crystal orientation of the sheet plane is the {110} plane and the crystal orientation of the rolling direction is parallel to the &lt;001&gt; axis. Grain-oriented electrical steel sheet is used for cores of transformers, generators, and other electrical machinery and devices.
Grain-oriented electrical steel sheet must have excellent magnetization and watt loss characteristics. The magnetization characteristic is defined by the magnitude of the magnetic flux density induced in the grain-oriented electrical steel sheet by a predetermined magnetic field. Here, B.sub.10 is used. Soft magnetic material having a high magnetic flux density, i.e., a good magnetization characteristic, can advantageously reduce the size of the electrical machinery and devices.
Watt loss is defined as power lost due to consumption as thermal energy in a core when it is energized by an alternating magnetic field having a predetermined intensity. Here, W.sub.17/50 is used. As is known, the watt loss characteristic is influenced by the magnetic flux density, sheet thickness, the impurities, resistivity, and grain size of the grain-oriented electrical steel sheet. Increased demand has arisen for grain-oriented electrical steel sheet having a low watt loss along with the trend toward energy conservation.
Grain-oriented electrical steel sheet is produced by hot-and-cold rolling a slab to the desired final sheet thickness and then finally annealing the resultant steel strip to realize selective growth of the {110}&lt;001&gt; oriented primary-recrystallized grains, i.e., to realize so-called secondary recrystallization. Controlling the secondary recrystallization, it is possible to increase the proportion of the accurately {110}&lt;001.degree. oriented grains in the crystal grains, thereby increasing the magnetic flux density of the grain-oriented electical steel sheet and, thus, reducing the watt loss. It is important to develop production techniques allowing control of the secondary recrystallization.
Japanese Examined Patent Publication (Kokoku) No. 40-15644 (Taguchi et al) and Japanese Examined Patent Publication (Kokoku) No. 51-13469 (Imanaka et al) disclose basic techniques for producing a grain-oriented electrical steel sheet having a high magnetic flux density and decreased watt loss.
The basic techniques disclosed in the above two Japanese examined patent publications however suffer from some fundamental problems. In the method dislcosed in Japanese Examined Patent Publication No. 40-15644, it is difficult to achieve overall optimum production conditions and to stably produce grain-oriented electrical steel sheets having a high magnetic flux density. As a result, the method is not appropriate for the stable production of products having the best magnetic properties.
The method disclosed in Japanese Examined Patent Publication No. 51-13469 involves double cold rolling and use of an expensive element, such as Sb or Se. This method therefore involves high production costs.
Also, both the prior art methods require high slab heating temperatures, disadvantageous from the viewpoint of the energy used for heating the slab, decreased yield due to slag generation and increased repair costs of slab-heating furnaces.
The need for high slab heating temperatures is related to the fact that to realize secondary recrystallization, fine precipitates, such as MnS and AlN, must be finely and uniformly dispersed in phases in the steel, while the steel is subjected to processes prior to the final high temperature annealing, so as to suppress growth of primary recrystallized grains having orientations other than the {110}&lt;001&gt; orientation during the final high temperature annealing (inhibitor effect).
In order to form such dispersion phases, when heating a slab to make it rollable, one must raise the slab heating temperature high enough to solid-dissolve MnS, and other inhibitor elements. These later precipitate as MnS, AlN, and the like, when the steel is hot-rolled or subjected to hot-strip annealing. The greater the degree orientation desired, the larger the amount of MnS, AlN, and other fine precipitates that must be present in the steel and, therefore, the higher the necessary slab heating temperature. Japanese Unexamined Patent Publication No. 48-51852 discloses an improvement of the method of Japanese Examined Patent Publication No. 40-15644. In this method, the Si content of the starting material is increased. A high silicon content however, narrowly restricts the conditions under which AlN can be ensured in the hot-rolled strip. Also, since the silicon content is high, the temperature range at which AlN precipitates during hot rolling in an appropriate manner for the secondary recrystallization shifts higher, requiring a higher slab heating temperature.
The adoption of continuous casting has created additional problems in the production of grain-oriented electrical steel sheet. In continuous casting linear, secondary-recrystallization-incomplete portions, referred to as streaks, are occasionally generated in the steel. This impairs the magnetic properties of the steel. The problem of streaks is greatly aggravated by a high Si content. Whe the Si content exceeds 3.0%, stable production of grain-oriented electrical steel sheet becomes extremely difficult. Japanese Unexamined Patent Publication No. 48-53919 (M. F. Littman) discloses to remove the problem of streaks by subjecting a continuously cast steel strand to double hot-rolling steps when producing a hot rolled strip. Japanese Unexamined Patent Publication No. 50-37009 (Akira Sakakura et al) discloses a method for producing grain-oriented electrical steel sheet wherein a hot-rolled steel strip is produced by double hot-rolling steps. These two prior art methods, however, do not fully utilize the advantages of continuous casting, i.e., omission of rough rolling. Two later publications, Japanese Unexamined Patent Publication No. 53-19913 (Morio Shiozaki et al) and Japanese Unexamined Patent Publication No. 54-120214 (Fumio Matsumoto et al), disclose how to employ single hot-rolling to produce grain-oriented electrical steel sheet using a continuously cast strand. These proposals, however, necessitate reconstruction of a casting or rolling installation and still do not completely solve the problem of streak generation.